1. Field of the Invention
The present invention relates to a method for producing tire using high strength lyocell dip cord, and more particularily, to a lyocell dip cord and tire produced by a method comprising the steps of: (A) dissolving 0.01 to 3 wt % of cellulose powder in portions in concentrated liquid N-methylmorpholine N-oxide (NMMO) to prepare cellulose-containing NMMO solution; (B) feeding the NMMO solution and cellulose powder into an extruder having a screw to be subjected to dispersing, mixing, shearing, kneading, melting and measuring ability in the extruder to prepare a swollen and homogenized cellulose solution; (C) spinning the cellulose solution through a spinning nozzle, passing the spinning solution through an air gap to a coagulation bath and coagulating the spinning solution to obtain a multifilament; (D) subject the multifilament to water-wash, drying and oil-treatment, followed by winding; and (E) twisting the wound yarn with a twisting machine to prepare a greige cord, weaving the greige cord and dipping the woven cord in a dipping solution.
2. Background of the Related Art
Various tire cord materials, such as polyester, nylon, aramid, rayon and steel, are currently used as a frame forming the inner part of a tire, but they do not satisfy perfectly various functions required in the tire cord. Basic performances required in such tire cord materials are as follows: (1) high tenacity and high initial modulus; (2) thermal resistance, and non-deterioration in dry and wet heat; (3) fatigue resistance; (4) dimensional stability; (5) excellent adhesion to rubber, and the like. Thus, the tire cord materials are used in applications according to their intrinsic physical properties.
For example, owing to low shrinkage and excellent dimensional stability in terms of intrinsic properties of fiber itself, as compared to polyester, a tire cord comprising rayon fiber has been mainly used in high-speed radial tires for automobiles which require good initial modulus (elasticity), thermal resistance, and dimensional stability among the above properties. The initial modulus is expressed by the slope of a load for bringing about extension to a given level and determined as the slope of an stress-strain curve obtained in a tensile test. A tire using a tire cord having a high modulus has effects to improve fatigue performance, heat generation, durability and the like, particularly handling stability of a radial tire, since it does not generate a little deformation under a load at a certain level. Particularly, a rayon cord does not show deterioration of physical properties in the temperature range (80˜120° C.) upon tire running in practice and thus shows good handling stability, as compared to other cord materials for automobile tire.
However, since conventional rayon tire cords have a little low tenacity and their modulus is extremely reduced by moisture absorption, it is difficult to control moisture and progress upon production of tires. Also, when they are formed into tires, if moisture permeates due to damage of the tire surface, the strength and modulus decrease and hence, tire performance is deteriorated. Accordingly, there is a demand for a tire cord having excellent tenacity and properties capable of maintaining strength and modulus upon moisture absorption which may occur during the production process.
Meanwhile, lyocell fiber, an artificial fiber composed of cellulose has low elongation and heat shrinkage, high tenacity and modulus, and thereby, excellent dimensional stability, as compared to rayon fiber. Also, it has low moisture content and thus, shows strength maintenance and a modulus maintenance of 80% or more in time of moisture absorption. Therefore, since it has an advantage of relatively small dimension change as compared to rayon (60%), it can be considered as an alternative for the above demand. However, there are some problems in spinning into a tire cord, as described below and thus, a tire cord using the lyocell fiber is not yet realized.
A process of preparing lyocell fiber using cellulose and NMMO as a solvent is widely used to produce articles made of cellulose such as film or fiber because the process is pollution-free process by recycling all the used solvent and the article such as film or fiber manufactured by the process has high mechanical strength. The process is disclosed in the U.S. Pat. No. 3,447,939 and so on.
U.S. Pat. Nos. 4,142,913, 4,144,080, 4,196,282 and 4,246,221 disclose a process for making a fiber in which cellulose is swollen in NMMO aqueous solution which contains less than 50% water, and then, the water in the resultant NMMO aqueous solution is distilled under reduced pressure so as to make the dope, followed by extruding into the fiber.
These processes, however, requires relatively long time so that the physical, chemical properties of the fiber made by the processes are deteriorated due to thermal-decomposition. Also, it consumes a lot of energy, thereby increasing production cost.
PCT WO 1994/06530 discloses a process for making cellulose solution by removing water using a thin-film evaporator. This process, however, has some disadvantages, in that an apparatus for implementing the process is too complicated and the production efficient is too low to make highly viscose cellulose solution.
U.S. Pat. No. 4,211,574 discloses a process for making fiber by swelling a cellulose sheet using liquid-state tertiary amine oxide containing 5 to 15 wt % water as solvent at 65 to 95° C., followed by agitating and heating, and then, spinning.
This process, however, couldn't obtain homogeneous cellulose solution due to a film formed on the surface of pulp sheet.
U.S. Pat. No. 4,416,698 discloses a process for making fiber by feeding solid-state NMMO (not liquid-state) and cellulose pulp into an extruder, followed by agitating them, and then, spinning. This process, however, is not suitable for a mass production because there remain a great amount of powder particles which are not dissolved in a solution by using two kinds of powder.
PCT WO 1997/47790 discloses a process for making fiber in which fibril type cellulose powder and high concentration NMMO aqueous solution containing 5 to 20 wt % water at 50 to 130° C. instead of a cellulose pulp sheet are fed into a twin-screw type extruder, followed by mixing and dissolving them, and then, spinning.
This process, however, has some disadvantages in that since during the spinning, there remains a great amount of power particles which are not dissolved and impurities in the resultant solution, and hence a filter for removing them must be very frequently replaced. It makes the cost of production too high because of changing filters too often. And a great amount of powder particles which are not dissolved in the solution couldn't obtain homogeneous cellulose solution, which makes physical and chemical properties of the obtained fiber deteriorated.
U.S. Pat. No. 4,416,698 and PCT WO 1997/47790 disclose a process for making cellulose solution through mixing, swelling (paste) and dissolving processes in an extruder. These processes, however, has a disadvantage in that they don't fully dissolve the cellulose.
The foregoing technologies have a lot of problems in reduction of energy and production of a highly homogeneous and highly viscose cellulose solution without unsolved particles.
Meanwhile fibers used in the tire cords or the industrial fields have their product quality determined by fiber properties such as tenacity and modulus, unlike the clothing fields, in which color development and handling properties are important.
For this tendency, fiber makers use various fiber production technologies to maximize properties of fiber and continuously improve fiber quality. Among various methods to improve fiber properties, by a structure having a polymer oriented along a fiber axis, it is possible to provide a fiber with excellent properties for clothing and industrial applications. Mostly, the orientation is achieved by drawing and the drawing step of various processing steps largely affects mechanical properties of a fiber.
In case of melt spinning, the drawing is carried out in a thermoplastic state in which a molecule shows good fluidity, while in case of solution spinning, the drawing is carried out by a wet or dry spinning method, after preparing a solution comprising a solvent and a polymer. For the dry spinning, the drawing is carried out while the solvent is being evaporated and for the wet spinning, the drawing is carried out mostly during coagulation according to the concentration of a coagulation liquid and temperature.
On the other hand, a spinning solution comprising three components of NMMO/water/cellulose which are commonly used for production of lyocell fiber is in a high temperature state of 80 to 130° C. Therefore, if the spinning is carried out by directly dipping a spinning nozzle in a coagulation bath like general wet spinning, it is difficult to attain sufficient drawing performance and properties due to rapid coagulation by solvent removal. Also, only dry spinning of a highly viscose cellulose solution of about 10,000 poise, the evaporation of the solvent cannot be achieved because NMMO is non-volatile.
There is proposed a dry-wet spinning method as a technology to improve physical properties and spinnability by maximally utilize an air gap between the spinning nozzle and the interface of the coagulation bath.
For example, U.S. Pat. No. 4,501,886 discloses a method for spinning cellulose triacetate using an air gap. Also, Japanese Patent Laid-Open No. Sho 53-81723 discloses a high speed spinning method of PAN fiber using an air gap, and U.S. Pat. No. 4,261,943 discloses to spray water as a non-solvent to an air gap in the range of 50 to 300 mm to prevent adhesion between filaments.
The foregoing technologies may increase degree of orientation of a fiber which is spun using an air gap. However, when they are directly applied to the production of lyocell multifilament for tire cords, there are factors making the process unstable such as adhesion between filaments due to increase of the number of filaments and it is thus difficult to realize a satisfactory spinning workability. Particularly, lyocell fibers obtained by the above methods show properties of tenacity and elongation which are not suitable for use as a tire cord.
Also, H. Chanzy et al. (Polymer, 1990 Vol. 31, pp 400˜405) have prepared a fiber having a tenacity of 56.7cN/tex and an elongation at break of 4% by adding a salt such as ammonium chloride or calcium chloride to a solution of cellulose of DP 5,000 dissolved in NMMO, followed by air gap spinning. It cannot be commercially used because of problems of collecting the coagulation liquid with the salt added.
According to U.S. Pat. No. 5,942,327, a fiber is prepared to have a tenacity of 50˜80cN/tex, elongation of 6˜25% and monofilament fineness of 1.5dtex by air gap spinning of a solution of cellulose of DP 1,360 dissolved in a NMMO hydrate. However, the number of filaments is only 50.
Considering that a filament for tire cords commonly comprises several hundreds of filaments to be more or less of 1,500 denier, it is judged difficult to attain properties required for a tire cord after twisting and dipping. In practice, for spinning of fiber, it is harder to control the condition of cooling, drying and washing in time of spinning of a thick denier fiber than a thin denier fiber. Thus, it is difficult simultaneously to realize properties of over a certain level and to maintain uniformity of individual filaments as a whole and consequently, only by examining properties of a fiber composed of 50 strands, it cannot be applied to a yarn for industrial application.
Also, the air gap spinning shows change in process stability and cooling efficiency for adhesion of filaments spun from a spinning nozzle in accordance with increase of the number of filaments. Therefore, there is a need for a new design considering the outer diameter of a spinning nozzle, orifice diameter, orifice gap, air gap length, conditions for supplying quenching air, running direction of coagulated liquid and drying conditions according to a spinning rate and the new design may cause change of physical properties.
In U.S. Pat. No. 5,252,284, 800 to 1,900 filaments were used and the spinning was carried out under conditions including a short air gap of 10 mm or less and a winding speed of 45 m/min. As a result, due to low draw ratio, it was found that elongation was as high as 15.4% and tenacity was at maximum 47.8cN/tex, which indicated that it is difficult to be used as yarn for tire cord in terms of tenacity and productivity.
Therefore, to solve the problems involved in the prior art, the present inventors has discovered that by dissolving a small amount of pulp to concentrated liquid NMMO, the solidification temperature of NMMO can be lowered, and consequently, the NMMO solution can be fed to an extruder at a relatively low temperature and the process temperature range can be expanded. Also, by the above effect, cellulose powder can be smoothly swollen with the NMMO solution at a low temperature and film formation on the surface of the cellulose powder can be prevented, whereby it is possible to prepare a homogeneous cellulose solution at a low temperature. Further, based on the above discovery, it is possible to prepare cellulose fiber with excellent tenacity by a dry-wet spinning method, which is suitable for a tire cord. Thus, the present invention has been completed.